论锥叠层石群(Conophyton)的形态发生——对苏北新元古代九顶山组一个似锥叠层石的剖析

ON MORPHOGENESIS OF CONOPHYTON STROMATOLITES ——ANALYSES OF A CONOPHYTON-LIKE SPECIMEN FROM NEOPROTEROZOIC JIUDINGSHAN FORMATION IN N. JIANGSU PROVINCE, CHINA

根据对苏北新元古代九顶山组似锥叠层石标本的剖析 ,初步揭示构成似Conophyton叠层石的微生物席可以分为两类。这两类席被作者称为“开端微生物席 (firstmicrobialmat)”和“继承微生物席 (succedentmicrobialmat)”。它们在整个叠层石发生和发育过程中起作不同作用。Conophyton叠层石的形态发生可能与开端微生物席的造型有着密切联系 ,这类微生物席在叠层石形成过程中起作生长“芽”或模具作用。继承微生物席仅仅起作叠层石的增高或增大的作用 ,它的发育经常受环境制约。

Abundant stromatolites have been found from the Neoproterozoic Jiudingshan Formation in Suining County, N. Jiangsu Province. Some of them have been partly silicified under condition of extremely early lithification, thus primary compositions and molds of microbial mats in these stromatolites are well preserved. The stromatolites are mostly stratiform, rarely small conical (Conophyton-like) and domed, with more or less clearly laminated structures. This paper describes the growth, movement, and particle-trapping behaviour of the microorganisms that built the Conophyton-like stromatolites and discusses morphogenesis of Conophyton. A systematic study of the stromatolites-micrbial mats in this region will be published elsewhere. The material studied here comes from the upper part of the Upper member of the Jiudingshan Formation, ca. 130 m above the boundary between Upper and Lower members at Liulou village, about 25 km northwest of Suining County (Text-fig.1). The unit consists of thin-bedded to medium-bedded, grey dolostones and thin-bedded grey siliceous limestones with partly silicified stratiform, small domed, conical and undulating forms of stromatolites. Abundant organic matter makes the chert dark or black in stromatolitic hand-sized samples. These stromatolites have been undergone silification, early diagenetic silica having replaced most of the carbonate of which they were originally composed. In the stromatolites chertification seems to start always in the dark laminae with a high organic content. Primary stromatolitic fabrics have been replaced by chert before neomorphic recrystallization. The chert is aphanitic and composed of chalcedony and microcrystalline quartz. In thin section, the black chert shows brown or amber in color, and the amorphous organic particles are disseminated throughout the silica matrix. The microbial mats were embedded mostly or partly in silica, and crystallization of the chert did not alter the form of the microbial mats. Cellular and filamentous structures within the mats are well preserved and fossilized. Most filamentous microfossils in microbial mats are preserved in their original growth position. Specimen No.Su-10 is a small Conohyton-like stromatolite. The stromatolite is commonly made up of dark cherts (silica) occasionally with some dolomite. The cone in this specimen is about 3-10 mm wide and 12 mm high, with prominent macrolaminae, frequent lenticular thickening in laminae (Plate Ⅰ-1). The laminae are mostly conical, and have considerably higher synoptic relief than those know from the modern mats. The axial zone recognized in Precambrian Conophyton stromatolites (Komar et al., 1965) is present in cone. Two morphological types of microbial mats have been recognized in longitudinal thin sections of cone. (1) Pinnacle-like Siphonophycus inornatum microbial mat (the present authors propose to term it first microbial mat) occurs in the base of stromatolite (Plate Ⅰ-2). It is constructed by a distinctive net-like complexity of growing dominated filaments, Siphonophycus inornatum, which possesses some trapped detrital grains (Plate Ⅲ-2). These filaments account for an overwhelming majority of the total community preserved in microbial mats and were densely packeted in mats of conical stromatolite (Plate Ⅲ-1). They are mat-builders. A few Myxococcoides sp. among coccoids were occasionally observed in the mat (Plate Ⅰ-3). They are probably planktonic algae that dropped into and were preserved with the microbial mat community. From the observations made on this stromatolite, the authors can speculate on how the first microbial mat of the developing stromatolite will be constructed. At first, filaments gliding on the substrate surface may encounter the bedrock irregularities and be deflected upwards above them, towards the light source (the sun). The erect tufts of filaments are initial accretions of this mat (Plate Ⅰ-4). The processes and mechanisms of erect growth of the filamentous tufts are expected to be similar to those described by M.R. Walter et al. (1976). Then a